Rheumatoid arthritis (RA) is an inflammatory joint disease which, untreated, results in progressive joint damage and disability.1 Early aggressive treatment offers the best chances of achieving good long-term outcomes in RA, but it remains a challenge to optimise early and accurate diagnosis of patients requiring such treatment.
The inflamed synovial membrane in RA has abundant new vessel formation; this is an early event and it has been shown to correlate with the clinical degree of synovitis.7,8 Furthermore, many of these vessels are immature, indicating a high degree of turnover which is likely to be contribute to increased vascular permeability.9 The integrin αvβ3 is expressed at low levels in health, but is up-regulated on activated vascular endothelial cells in angiogenesis as well as on activated macrophages and osteoclasts,10,11 both of which accumulate in the inflamed joint.12,13 Expression of αvβ3 is markedly upregulated on synovial vascular endothelial cells in both seropositive and seronegative arthritis.14,15 
Over the last decade high-resolution ultrasound (US) of joints, particularly when combined with power Doppler (PDUS) assessment, has been increasingly employed in the assessment of inflammatory arthritis. Evidence of inflammatory activity can be seen in clinically uninflamed joints,2 indicating that US has superior sensitivity to clinical examination. Furthermore, the presence of active synovial inflammation as determined by power Doppler signal has been shown to be predictive of joint damage.3,4 PDUS images vascular activity in the synovium and has been shown to correlate with histologically determined synovial vascularity5 and inflammation.6 
There is a substantial body of evidence that vascular imaging is a valuable predictive tool in RA for persistence of inflammation and for joint damage. PDUS has been used as a surrogate for vascular imaging, but the technique is limited in terms of its accessibility. Despite its advantages, US is available in only a relatively small number of rheumatology departments. It is time-consuming and therefore in daily practice assessment is limited to a few joints; thus comprehensive imaging of all joints is impractical in routine clinical practice. Furthermore, US is expensive in terms of resources and the number of rheumatologists with the requisite skills remains limited. A practical alternative method to US for imaging arthritis is thus needed.
Magnetic Resonance Imaging (MRI) has a similar sensitivity to US for joint inflammation. However, using MRI it is impractical to image more than a few joints at a time. MRI is also extremely expensive. MRI therefore has a limited role in the routine assessment of inflammatory arthritis, and is not a viable alternative to US in most cases.
Positron emission tomography (PET) has been suggested as an alternative approach for imaging arthritis. For example, a recent study demonstrated some specificity of uptake of a 68Ga-labeled dimeric RGD peptide for PET imaging in RA.22 However, the study demonstrated limited correlation between the results of the PET imaging and clinical assessment. Furthermore, PET imaging does not have potential for routine clinical practice due to the high cost and restricted accessibility of PET equipment. PET is thus not a practical alternative to US or MRI for imaging RA.
Various tracers have been investigated for use in nuclear imaging, and both 99mTc- and 18F-labelled compounds are being validated for the imaging of neoangiogenesis in cancers.19-21 However, to date, a suitable method of imaging arthritis by nuclear imaging has not been demonstrated. A practical alternative method to (PD)US, MRI and PET for imaging arthritis is thus needed.